This ASUS replacement battery pack is a 4000mAh, 3.8, rechargeable, removable, Li-Polymer battery designed to work with ASUS AST21 1B. The Hi-Quality battery meets or exceeds original manufacturer specifications for the ASUS AST21 1B Laptop Battery in quality, durability, and performance. Li-ion batteries do not suffer from the memory effect. They generally last between 300-500 charge/discharge cycles. All laptop batteries are tested for 100% quality control assurance. This battery for ASUS AST21 1B includes a one year warranty. We offer a 30 day money back guarantee and fast shipping on all of our products.